Generally, where polymerizable liquid monomers are polymerized in the presence of a suitable polymerizing catalyst, there are two different cases, namely, one wherein the polymeric mass produced is soluble in the monomers and therefore the mass forms a viscous liquid mass, which in turn is caused to form a highly viscous material as polymerization progresses, and the other wherein the polymeric mass is insoluble in the monomers and accordingly, as polymerization progresses, there occurs a change of phase such that the mass undergoes a change from liquid state into slurry state before it is finally separated out in the form of a completely solid polymerization product. The invention relates to a system of the latter case. A typical example of continuous manufacture of a solid polymerization product through such polymerization reaction as involved in the latter case is polyvinyl chloride production through bulk polymerization, and another is polyacetal resin production through bulk polymerization In the following description, therefore, a method of manufacturing polyacetal resin is mainly taken up.
It is already known to produce a polyacetal resin by using such cation active polymerizing catalysts as boron trifluoride, phosphorus pentafluoride, tin tetrachloride, perchloric acid, or salts thereof or complex salt and through monopolymerization of trioxane or through copolymerization with cyclic ethers such as trioxane and ethylene oxide, or with cyclic formal or the like. Such method is in actual practice on an industrial scale. In the process of such polymerization or copolymerization, as already stated, so-called phase change takes place as polymerization progresses, so that the polymeric mass changes into a solid polymeric mass via a short period of its slurry state. Further, in a so-called bulk polymerization method wherein virtually no diluent is present, reaction is so fast that the phase change is very abrupt; hence, it is no easy task to control the reaction. For example, if such polymerization or copolymerization reaction is carried out in static condition, a large bulky and tough product is produced in an instantaneously short time, so that material handling in the subsequent crushing, cleaning, and refining stages is extremely difficult. Moreover, because of internally accumulated polymerization heat, temperature control is almost impossible, with the result of quality deterioration and unfavorable rate of conversion. Therefore, in view of such specific aspect of reaction and in order to prevent formation of such bulky polymerization product and to allow efficient production of a fine-particle polymerization product which is comparatively quality-stable, a number of approaches have been proposed. One basic concept common to those approaches is utilization of an extruder type polymerization reactor having a two-parallel-shaft agitator arrangement.
The concept of utilizing such extruder type reactor having a two-parallel-shaft agitator construction for the purpose of manufacturing polyacetal resins, pioneered by a proposal to use a two-shaft screw type extruder as disclosed in Japanese Published Examined patent Application Nos. 47-629 and 47-42145, and a proposal to employ a two-shaft mixer consisting of a combination of screws and elipsoidal disc paddles, with subsequent development and improvements, led to a number of proposals including Japanese Published Unexamined patent application Nos. 53-86794, 56-38313, and 58-32619 - 21. There are two types, one such that the parallel two shafts rotate in same direction and the other such that the shafts rotate in opposite (different) directions, both having similar functions. The former type is claimed as having good self-cleaning characteristics; and in the area of the latter type, there has been proposed an arrangement such that shear force is variably effected automatically in a desired direction according to the phase change, as disclosed in Japanese Published Unexamined patent application. At present, manufacturing techniques employed for industrial production of polyacetal resin are largely based on said proposal.
Recently, the demand for polyacetal resins has been constantly on the increase, and in view of the fact that high quality is demanded of the resin in thermal stability characteristics in particular, the state of the art for production process is not always satisfactory. Further efforts are desired for improvement of polymerization product yield or conversion rate per unit of equipment, and also for improvement in quality of polymerization products through more effective stabilization treatment. When a two-parallel-shaft rotary agitator type reactor incorporating various features based on aforesaid inventive proposals, with modified paddle configuration and arrangement, is employed in producing polyacetal resins, it is indeed possible to obtain a polymerization product of a relatively fine particle form at a high conversion rate, if the equipment is of a small type of laboratory scale. However, as the size of the installation becomes larger, results obtained are not necessary satisfactory. For example, a polymerization product of coarse-particle size, e.g., small-finger size, rather than fine-particle size, is produced in a larger proportion. Further, a thicker scale of polymeric matter is likely to deposit on the inner wall of the equipment, thus causing a decrease in heat transfer efficiency, which inevitably results in decreased conversion rate and lower product quality. In such case, the polymerization product from the reactor is often ground and refined by a grinder, and then led to a so-called quenching stage in which it is subjected to stabilization treatment, such as cleaning, under good cooling condition, before the product enters the subsequent stage. Where operation is carried out continuously, masses of polymeric product being continuously turned out involve considerable fluctuations in quality and furthermore the flow in the process of polymeric masses converted from the stock is subject to considerable pulsation; therefore, attempts to control reaction are not always effective to the satisfaction of the accuracy requirements.
The functions required of a two-shaft agitator type reactor of the kind include:
(a) fast and homogeneous blending of catalists and polymerizable monomers, i.e., trioxane or cyclic compound monomers copolymerizable with trioxane;
(b) prevention of polymeric particle adhesion at an initial stage where a polymeric product is separated out as polymerization initiates, that is, at a stage where the polymeric product is in slurry condition (high shear force);
(c) prevention of solid polymeric product from bulking, or grinding of bulk polymeric product (high shear force);
(d) retention of within-reactor self-cleaning characteristics (prevention or removal of deposit retention);
(e) retention of good efficiency of heat transfer to fine-particle polymerization product (retention of the specified polymerization temperature, relaxation or reduction of reaction temperature internal build-up); and
(f) prevention of product flow from pulsation throughout the process of from stock introduction and to product discharge.
Even if various improvements are made in paddle configuration and/or paddle arrangement, it is practically impossible that all these requirements are fully satisfied within one reactor. The reason for aforesaid difficulty of sealing up reactors of the type lies in this point. More especially, it must be noted that phase change from the liquid to the solid is abrupt because the speed of polymerization reaction is very fast, from which fact it follows that the density change involved is considerable and that the reactor contents also undergo considerable volumetric change. As a result of such phenomena, the fill level of reactor contents varies locally in the axial direction; therefore, pulsation is caused to the flow of product, which leads to fluctuations in agitation or grinding efficiency, and ultimately to a lower conversion rate and lower product quality. In order to overcome these problems, measures are often taken to regulate the flow by adopting a suitable combination of "feed" type and "reverse (back feed)" type paddles in paddle arrangement. However, such approach may often involve troubles, such as excessive power consumption, overloading, and reactor content blocking, during operation, and is not always useful for adequate regulation purposes. In Japanese Published Unexamined patent application No. 53-86794, it is proposed to divide the polymerization process into two stages, that is, a first stage in which a self-cleaning type reactor is employed, and a second stage in which a pin-mixer type heat-exchange reactor is employed. However, the proposal gives no basic measure for overcoming the difficulties involved in the first-stage reactor.